In switching DC-to-DC converters, such as e.g. buck converters, an auto-sleep function decides whether the switching converter should be operating in a low output current/low quiescent mode (in buck converters known to the inventors this is referred to as PFM mode) or a high output current/high quiescent mode (PWM mode).
The low current mode is very efficient but cannot handle high load currents. The high current mode gives good load transient performance, but with high quiescent currents.
The buck converter therefore automatically switches between modes. This is performed by measuring output current and deciding which mode would give higher efficiency.
The buck converter measures in prior art the output current by measuring the voltage drop across the PMOS pass-device. This voltage drop is very small, but is proportional to the current in the PMOS pass device while it is on. Of course, the PMOS pass device is not switched on all the time, and for some parts of a cycle it will be switched off. At this point the switching node goes low and, if the voltage across the PMOS pass device is still measured, it would correspond to a very large, and false, output current.
Measuring another voltage than the voltage drop of the PMOS pass device is not practical in most buck converter or other converter architectures.